The present invention relates to frequency demodulation circuits, particularly for the field of color television and more especially for demodulation of the color subcarrier wave in SECAM receiving apparatus. It applies more particularly to control of the slope of the demodulator so as to do away with the adjustments, the thermal and time drifts and the manufacturing dispersions which affect the quality and the fidelity of reproduction of colors in SECAM receivers.
The problem resides in an antagonism between the results of manufacturing demodulators, especially with integrated technology, which result in a dispersed and unstable demodulation slope and the requirements for using these demodulators in SECAM television receivers which require a constant and stable slope. It is in fact imperative to restore accurately the DC component of the signal.
A solution of the prior art consists in using tuned circuits for constructing a demodulator of the FOOSTER SEELEY type, for example. This solution is not satisfactory for the following reasons: cost of the tuned circuits and especially cost of the adjustments, thermal and time drifts of the coils and condensers (despite trimming), large number of connection terminals between the integrated circuit and the external components.
Another solution of the prior art consists in forming a demodulator by means of a phase-control loop (oscillator+phase comparator). This solution, which constitutes nevertheless a progress with respect to the preceding one, is affected by the same type of defect: the demodulation slope depends on the components which determine the natural frequency of the oscillator. The precision and the stability of these components, even placed outside the integrated circuit, are insufficient, resulting in the need for adjustments. This dispersion over the demodulation slope has the further disadvantage of decentering the demodulation range with respect to the band to be demodulated. This demodulation range must then be increased so as to make sure that it contains the band of frequencies to be demodulated in the worst case of dispersion. But, increasing the demodulation range presents disadvantages: it is in fact known that in a phase-control loop, the increase in the pickup range is obtained by an increase in gain in an open loop, which makes the device more difficult to damp in a closed loop. The phase-control loop risks furthermore locking on parasite signals outside the effective band of the signal to be demodulated.
Among other attempts for doing away with adjustments and drifts, the sampled control of the response of the demodulator by the line-bursts or wave trains transmitted by the emitter at the beginning of each line may further be mentioned. The frequency of these line-bursts corresponds in fact to the level of the "black" (absence of color).
This latter arrangement is not satisfactory either for the following reasons:
the line-bursts only last 3.5 to 4 .mu.s and their beginning cannot be used; in fact, in the first part of the line-bursts the demodulator is operating under transitory conditions. The sampling must not finish too late and encroach on the signal which follows the line-bursts. In practice, for good reliability, the sampling time will have to be less than 2 .mu.s and accurately centered on the rear part of the line-bursts which involves a special circuit, not free from adjustments;
the line-bursts are affected by the propagation conditions, particularly by the parasite echoes which may induce interferences with the synchronizing "blips" which results in an incorrect demodulated level for achieving accurate control.
Moreover, in the SECAM system, the subcarrier wave, between 3.8 and 4.8 MHz, taken from the carrier wave, provides chrominance information in the form of two chrominance signals called B--Y and R--Y. The subcarrier wave is modulated in frequency and the colors are well reproduced if the output voltages of the demodulators are indeed indexed to the corresponding subcarrier frequencies delivered by the emitter, i.e. if the slope of the demodulators is well-defined and stable.
Similarly, when the emitter sends out nonoperating frequencies (corresponding to the absence of color) F.sub.OR for the red chrominance signal and F.sub.OB for the blue chrominance signal, the two demodulators must deliver a well-defined voltage for the grays to be pure and not colored.